I did drove nearly 150km last evening/night, every place I remember being flat is not really flat, there are small up and down mini hills or then straight is not long enough.
I did found one place which gave me 50 seconds of coasting, still having small amount of uphill at both ends, but rather small I did though, did plenty of runs, put data to spreadsheet and I get negative cd, of course everytime there is downhill when I start the run, to both ways and uphill at the end.
For me it looks like that getting cd reliably measured it might cost so much in fuel that it would be rather pointless. Trying to find suitable road has already cost more than mods will cost and based on last summer's experience, I doubt that I can very easily save enough money to get even these costs covered with low mileage I do, so maybe it is best to forget testing and just apply aerodynamic aids and do hill experiment on known place at known weather and speed to estimate coasting abilities.
Wet road conditions change coasting dramatically, I'm barely managing to keep 75kph (starting speed 75kph) on one steep downhill, where I remember last summer speed being easily around 110kph with mods (starting speed 65kph), without mods and at dry weather I was above 90kph (starting speed 65kph).
With driving over 150km in search of good piece of road, I found out also 2 rabbits and 1 truck, nothing else moving out there.
Edit: I have been thinking a bit of Kammback, it will be quite small as there are no large space between end of roof and end of bumper and even it might be illegal to put any kind of kammback, it is at least less likely to be major offence when it is within body lines.
I have been reading Milliken's Race Car Vehicle Dynamics for other purposes, then I have been reading Aerohead's posts and generally researching this subject more.
I'm thinking this kind of bit agressively angled design, because of short distance and relatively slow speeds I use, but now I'm not sure if I should use flow separator at edge of Kamm to prevent vortex behind the car? I plan to fill back of car from air that runs from sides and under the car, but I'm afraid that without flow separator it might cause big vortex behind the car under the Kamm.
Red bit is one I wonder if I should put in there or not. I can of course make it detachable so that I can test with it and without it, but guessing before making it is always fun
Vekke posted link to pdf in Aero section where A2 Audi got good cd reduction from small straight pieces added to rear when they did lower angle of diffusor, that is something to think about too, but I doubt that I can attach lot to bumper, also tail light might get blocked when viewed from side and that is something to avoid.
Bellypan and wheel spoilers might be ones to adopt from that however and rear wheel arches I have already planned to block, front wheel arches would be great to block too, but it is not very easy to do well, however there is lot to be gained as I remember how at front even small tire arch gap reduction made noise a lot less.